The metallocene catalyst system is a combination of a main catalyst consisting of a transition-metal compound having a central group 4 metal and a co-catalyst consisting of an organometallic compound having a group 13 metal such as aluminum as a principal component. Due to its single-site catalytic characteristic, the metallocene catalyst system can be used to prepare polymers such as polyolefins with narrow molecular weight distribution.
The molecular weight and the molecular weight distribution of polyolefins are critical factors that determine the physical characteristics, and the flowability and mechanical characteristics affecting the workability of the polymers. For fabrication of various polyolefin products, it is important to enhance the melt workability of the polymer by controlling the molecular weight distribution of the polymer. In particular, toughness, strength, environment stress cracking resistance, or the like are of a great importance for polyethylene. Thus, there has been suggested a method of enhancing the mechanical properties and the workability in low molecular weight moiety of resins with high molecular weight by polymerizing polyolefins having bimodal or wide molecular weight distribution.
On the other hand, the industrial techniques for synthesis of polyolefins are classified into high pressure process, solution process, slurry process, and gas phase process. The development of metallocene catalysts has driven many attempts to produce different polyolefins by varying the type of the metallocene catalyst according to one of those techniques. The polymerization techniques most frequently used with metallocene catalysts are solution process, gas phase process, and slurry process. Among these, the gas phase process or the slurry process requires it to control the bulk density of the polymer to a great extent for the sake of increasing the yield per volume of the polymerization reactor and to prevent reactor fouling associated with polymer deposition on the wall of the reactor for continuous operation of the reactor. The most popular method of increasing the bulk density of the polymer such as polyolefin and preventing reactor fouling is immobilizing a homogeneous metallocene catalyst on a solid carrier such as silica or alumina. In many cases, however, the use of a supported catalyst for polymerization of polyolefin to increase the bulk density of the polymer may cause deterioration of the catalytic activity.